Speed up property tree clip map caching

This looks like a ~3% win on Linux for cdp speed by removing the
heap allocation and storing cached vectors local to the clip node.
I don't know if this will fix the entire regression, but seems
faster.

Other things tried locally were having a global map and a global
vector of clips, and neither of these were huge wins.

Bug: 708252
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I8df7b92535c010cf21de56199bf3f731cc004b44
Reviewed-on: https://chromium-review.googlesource.com/506467
Commit-Queue: enne <enne@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#477138}

• cc/trees/clip_node.cc[diff]
• cc/trees/clip_node.h[diff]
• cc/trees/draw_property_utils.cc[diff]
• cc/trees/property_tree.cc[diff]